The present invention relates to an antenna device to be equipped to a movable body such as an automobile for receiving electric waves.
An automobile is generally equipped with an audio system such as car radio or car stereo system and, recently, a car television system or car telephone is also equipped. Furthermore, a global positioning system (GPS) as a satellite orbit utilizing system which determins a present position of a moving body by receiving an electric wave from an artificial satellite has come into practical use and, in certain cases, such GPS is equipped for a movable body such as an automobile or a craft.
In a case where such equipment is provided for the movable body to receive or transmit the electric wave, a fading phenomenon may be caused in accordance with the movement of the movable body, and in order to reduce the influence of the fading, it will be necessary to perform a diversity receiving process, which is a method in which more than two kinds of receiving signals having identical modulation signals and informations and having different signal-to-noise ratios at an optional instance are utilized selectively or in combination thereby to obtain receiving signals to be demodulated. It is therefore necessary to locate a plurality of antenna device capable of receiving different electric waves respectively for receiving signals by the utilization of such diversity receiving methods.
For example, with respect to a conventional antenna device for a car radio system equipped with an automobile, a rod antenna element is attached in a standing manner at a rear portion of the automobile and a defogger provided for a rear window is commonly utilized as an antenna element. In particular, the diversity signal receiving is carried out by the defogger arranged substantially horizontally and the rod antenna element, and a switching circuit is also equipped for selectively switching the rod antenna element and the defogger for receiving the electric wave from selected one of these antenna elements in good condition. The signal selected by the switching circuit is then tuned by a tuner and, thereafter, amplified by an amplifier. Thus, the signal is outputted as a sound through a speaker.
However, in the conventional antenna device, since one rod antenna element and a defogger, as a horizontal antenna element, are utilized in combination as an antenna device, it is difficult to always receive an electric wave in good condition by selectively utilizing the switching circuit. Moreover, there is a problem for preliminarily setting the antenna characteristics of the rod antenna element and the defogger, respectively.
In another aspect, as described before, the GPS as a satellite orbit utilizing system which determines a present position of a moving body by receiving an electric wave from an artificial satellite has come into practical use and, in a certain case, such GPS is equipped for a movable body such as an automobile or a craft. Furthermore, there is a problem of the conventional antenna device in which the rod antenna element attached in a standing manner from the automobile, for example, may be broken or folded by mischief at a time when the car is parked.